I am trying to relax the sample at zero pressure for 200 ps. I used fix NPT command for this.
But, instead of decreasing the potential energy, the potential energy is increasing. I have copied here part of my input file. Any comment would be appreciated.
Without knowing anything about your specific case, I could imagine that if your initial structure is very stable it can have negative pressure (due to the virial contribution) and therefore increasing the pressure to 0 with a barostat at a fixed temperature can in fact increase the potential energy.
Thank you for the comment. I appreciate your help. I did check the pressure and it is positive. I am sorry that I did not provide sufficient information.
My sample was initially at 0k. Since I applied fix NPT right after increasing the temperature to 900K using the Velocity command. Now, I think that the reason for increasing potential energy.